The present invention relates to a method an equipment for writing a Bragg grating, particularly an apodized Bragg grating, in a waveguide. In the course of the present description, reference will be made to optical fibres, but this reference should be understood as providing an example rather than being limiting, since the technology described is equally applicable to waveguides in integrated optical systems.
Typically, the optical fibres used for telecommunications are doped with germanium, which induces a property of photosensitivity to UV radiation. In order to write a Bragg grating in an optical fibre, this property is used to modify the refractive index locally by means of UV illumination.
As is known, an in-fibre Bragg diffraction grating is a length of fibre which has an essentially periodic longitudinal modulation of the refractive index in its core. This structure has the property of retroreflecting the light in a wavelength band centred on the Bragg wavelength. The Bragg wavelength, as is know (for example, from Report 3.3 in the publication xe2x80x9cFiber Bragg Gratingsxe2x80x9d, by Andreas Othonos and Kyriacos Kalli, Artech House, Boston/London, 1999) can be expressed as follows:
xcexB=2xc2x7neffxc2x7xcex9xe2x80x83xe2x80x83(1)
where neff is the effective refractive index and xcex9 is the spatial period of the diffraction grating.
Additionally, as is know (from Report 3.4 of the aforesaid publication xe2x80x9cFiber Bragg Gratingsxe2x80x9d, for example), in the most general case the variation of the refractive index n along the axis z of the fibre core can be expressed by the following relation (which shows all the possible dependences of the variable z):
n(z)=n0(z)+xcex94n(z)sin(2xcfx80z/xcex94(z))xe2x80x83xe2x80x83(2)
where n0(z) is the mean local value of the refractive index and xcex94n(z) represents the local envelope of the refractive index. The effective refractive index neff of relation (1) is proportional to the mean refractive index n0(z) of relation (2) by a term defining the confinement (typically indicated by xcex93) of the fundamental mode of the fibre.
Uniform gratings, gratings known as xe2x80x9cchirpedxe2x80x9d, and apodized gratings are known and can be distinguished by the variation of the refractive index.
In uniform gratings, the terms n0(z), xcex94n(z) and xcex9(z) are constant, as shown in FIG. 1a, which shows the typical variation of the refractive index n (normalized to 1) as a function of the co-ordinate z (expressed in arbitrary units). Additionally, as shown in FIG. 1b, the reflection spectrum of a uniform grating typically has a central peak at the Bragg wavelength and a plurality of secondary lobes. These secondary lobes can be disadvantageous is some applications, for example when the Bragg grating is used to filter a channel (at a corresponding wavelength) in a multi-channel optical transmission system. This is because, in this case, the secondary lobes of the reflection spectrum introduce an undesired attenuation into the transmission channels adjacent to those which are to be filtered.
In apodized gratings, the term xcex94n(z) is variable, and the refractive index n(z) has a variation of the type shown qualitatively in FIG. 2a (in which n is normalized to 1 and z is expressed in arbitrary units). The refractive index therefore shows an envelope corresponding to a predetermined curve. A typical variation of the reflection spectrum of an apodized grating is shown in FIG. 2b. It is clear that a suitable modulation of the term xcex94n(z) enables the secondary lobes to be reduced around the principal reflection peak. A grating of this type can therefore be used advantageously for channel filtering in a multi-channel system, thus reducing the aforesaid problem of the attenuation of the channels adjacent to the filtered channel.
In chirped gratings, one or the other of the terms n0(z) and xcex9(z) is variable. Owing to this variability, and since the Bragg wavelength is proportional, for the reasons stated above, to the product of n0(z) and xcex9, the reflection bands of these gratings are wider than those of uniform gratings. FIGS. 3a, 3b, 3c show, respectively, the qualitative variation of the refractive index in the case in which the term n0(z) is modulated, the variation of the same parameter in the case in which the term xcex9(z) is modulated (with a continuous variation from approximately 500 nm to approximately 502 nm, for example), and the typical reflection spectrum of a chirped grating. As can be seen in the spectrum of FIG. 3c, the reflection peak is considerably widened. A grating of this type can therefore be used as a wide-band reflection filter or, more typically, as a chromatic dispersion compensation device. If the term xcex94n(z) is also modulated, the grating bercomes a chirped apodized grating.
There are various known techniques for writing an apodized Bragg grating. In these techniques, the fibre is exposed to suitably shaped UV interference fringes, to produce a corresponding variation of the refractive index, and particularly of the local envelope xcex94n(z).
The known techniques essentially fall into two categories: interferometric techniques and phase mask techniques.
Interferometric techniques essentially consist in dividing a UV beam into two components and making them strike the fibre at a predetermined relative angle, thus generating the interference fringes which induce the desired variation of the refractive index. These techniques are highly versatile, since by varying the relative angle between the two components it is possible to vary the parameters of the grating, particularly its period.
However, interferometric techniques are poorly suited to serial production, since the xe2x80x9cset-upxe2x80x9d for writing is particularly sensitive to external factors (temperature, vibrations, etc.), so that the parts require frequent realignment. The application of these techniques is therefore essentially limited to the research field.
The phase mask techniques are generally considered more suitable for large-scale production, owing to the high repeatability, the lower susceptibility to external factors, and the fact that the UV beams require a lower coherence length.
A phase mask is a quartz substrate on whose surface there is a series of rectilinear projections running in a principal direction and parallel to each other, and forming, in section, a profile which is essentially of a square wave type. These projections are typically equally spaced and of equal height in a uniform mask, at variable intervals in the case of a chirped mask, and of variable height in an apodized mask.
To write the grating, the phase mask is usually positioned facing the portion of fibre concerned, and orientated in such a way that its principal direction (as defined above) is parallel to the fibre axis. When the UV radiation passes through it, the phase mask generates at its output interference fringes with an essentially sinusoidal variation and with a period xcex9 equal to half of the period xcex9m of the projections of the mask. In greater detail, the electromagnetic radiation leaving the phase mask can be subdivided into different orders m associated with corresponding propagation angles xcex8m according to the relation                               sin          ⁢                      xe2x80x83                    ⁢                      θ            m                          =                  m          ⁢                      λ            Λ                                              (        3        )            
The aforesaid fringes are generated from the orders +1 and xe2x88x921 (values+1 and xe2x88x921 of m), while the other orders, particularly the zero order, are unwanted, since they tend to diminish the visibility v of the fringes. The visibility v is defined, in a first approximation, by the following relation:                     v        =                                            I              max                        -                          I              min                                                          I              max                        +                          I              min                                                          (        4        )            
where Imax and Imin are, respectively, the peak intensity and the valley intensity of the fringes.
In general, phase masks are designed in such a way as to reduce (typically by approximately 1% to 3%) the transmitted zero order and to maximize (typically with an increase of approximately 30% to 40%) the quantity of light of the orders +1 and xe2x88x921.
The article by J. Hxc3xcbner, M. Svalgaard, L. G. Nielsen and M. Kristensen, xe2x80x9cPhenomenological Model of UV-induced Bragg Grating Growth in Germanosilicate Fibersxe2x80x9d, SPIE Vol. 2998, describes a model for the growth behaviour of Bragg gratings in a germanosilicate fibre when the phase mask technique is used. The authors of this article conducted experiments in which the fibre was illuminated with an excimer laser (of the ArF or KrF type) or a doubled frequency argon laser (FRED). In the case of the excimer laser, the laser beam was shaped by means of two cylindrical lenses in such a way that it had a cross section whose dimensions were equal to the length of the grating to be written, the fibre was positioned centrally with respect to the beam and the phase mask was positioned in contact with the fibre. In the case of the FRED laser, the beam was shaped by means of a cylindrical lens on a line parallel to the axis of the fibre and the phase mask was positioned at approximately 100 xcexcm from the fibre.
Some known techniques for writing apodized gratings by means of phase masking are described below.
A first technique consists in the use of apodized phase masks. Apodized phase masks are masks with a variable diffraction efficiency. In this technique, by contrast with the normal situation, use is made of variations of intensity of the beams of the orders xe2x88x921 and +1 to modulate the visibility of the fringes along the fibre axis according to the desired apodization profile. The article by Albert et al., xe2x80x9cApodisation of the spectral response of fibre Bragg gratings using a phase mask with variable diffraction efficiencyxe2x80x9d, Electronics Letters, vol. 31, no. 3, 1995, pp. 222, 223, describes the production of apodized gratings by this technique. In particular, this article proposes the production of apodized masks in which the depth of the projections is variable.
The applicant considers that this technique, although it provides a relatively high degree of repeatability and a relatively simple production process, is very inflexible in respect of the apodization profile that can be obtained, since this is fixed by the particular shaping of the phase mask. The applicant has also noted that apodized phase masks are very expensive and more subject to damage than other types of mask.
A second technique, described in the article by Malo et al., xe2x80x9cApodised in-fibre Bragg grating reflectors photoimprinted using a phase maskxe2x80x9d, Electronics Letters, vol. 31, no. 3, pp. 223-225, 1995, consists in the production of an apodized grating by a double exposure which makes it possible to obtain an essentially constant mean value of the refractive index. The first exposure is carried out with the use of an amplitude mask in which the transmissivity along its principal axis is variable according to the desired apodization profile, and the subsequent exposure is carried out with the use of a second amplitude mask whose transmissivity is complementary to the first.
In this case also, since a specific amplitude mask is required for each type of apodization and for each grating length, the applicant considers that the proposed technique has little flexibility.
International patent application WO 98/08120 in the name of Pirelli Cavi e Sistemi S.p.A. describes a technique called the xe2x80x9cContinuous Fiber Grating Techniquexe2x80x9d, in which a fibre exposed through a mask to a UV radiation modulated periodically over time, is translated continuously along its axis (by means of a slide controlled by an interferometric system) in such a way that successive exposures produce superimposed fringes. This technique enables an apodized grating to be produced by causing an oscillation of the position of writing on the fibre.
This technique, while providing a high degree of flexibility in respect of the apodization profiles which can be obtained, requires expensive equipment.
The article by Cole et al., xe2x80x9cMoving fibre/phase mask-scanning beam technique for enhanced flexibility in producing fibre gratings with uniform phase maskxe2x80x9d, Electronics Letters, vol. 31, no. 17, pp. 1488-1489, 1995, describes a technique for writing apodized gratings providing the scanning of a laser beam parallel to the axis of the fibre and its passage through a phase mask facing the fibre, and also a relative movement between the fibre and mask parallel to the axis of the fibre. The velocity of movement of the fibre relative to the mask is much lower than the laser beam scanning velocity. By controlling this relative movement, it is possible to produce chirped, apodized and phase step gratings.
U.S. Pat. No. 5,912,999 describes a method for writing relatively long gratings. In this technique, the fibre is moved longitudinally at controlled velocity relative to the mask, and the illuminating laser beam is amplitude modulated in order to produce apodized gratings.
With regard to this technique, the applicant considers that the modulation of the energy induces a modulation of the mean refractive index n0(z) and, consequently, the growth of undesired lobes at wavelengths shorter than the central wavelength.
EP 0684491 describes a method of writing Bragg gratings, in which the fibre is illuminated with interference fringes generated by the passage of electromagnetic radiation through a phase mask orientated in such a way that its diffracting elements are inclined at a predetermined angle (typically a right angle) to the axis of the fibre. During writing, the distance between the phase mask and the fibre is progressively varied, by means of a piezo-electric device, with a ramp variation. This relative movement, of the order of some tens of micrometers, is provided in order to reduce the sensitivity of the writing of the grating to the distance between the phase mask and the fibre.
The applicant notes that this method does not provide any apodization of the grating. The applicant also notes that the movements provided by this technique in other words movements up to a maximum of 50 xcexcm, are not sufficient to significantly modify the visibility of the fringes. This movement therefore only permits an average operation of the interference fringes.
International patent application WO 00/02068 describes a grating formed in an optical waveguide having a photosensitive core. A beam of actinic radiation, suitably shaped and filtered, is used to generate, by means of an optical system comprising a beam splitter, mirrors and lenses, or by means of a lens and a phase mask, two optical beams which are inclined with respect to each other and which interfere with each other on the photosensitive core. The peak intensities of the interfering beams are spaced apart along an optical axis of the waveguide, to reduce the lateral lobes of the spectral response of the grating by a smoothing of the mean refractive index. A second exposure with the two beams, but without the effect of interference between the beams, causes a further smoothing of the refractive index.
The applicant considers that the modulation of the refractive index obtainable by this writing process is significantly dependent on the shape of the optical beam which is generated, and on the characteristics of the writing device, and this technique therefore has limited flexibility. In particular, the applicant considers that this technique may require a hardware modification of the grating writing equipment in order to produce gratings of different length or with a different apodization profile. Moreover, since essentially the whole of the actinic radiation beam generated by the laser source is used in this process, the non-uniformities and asymmetries of the beam can affect the resulting grating.
The applicant has tackled the problem of providing a technique for writing a Bragg grating, particularly an apodized Bragg grating, in a waveguide, by means of which gratings having desired spectral characteristics (in other words gratings which have a predetermined variation of the refractive index) can be generated with a high process flexibility and high repeatability, without requiring highly precise alignment or expensive equipment.
The applicant has found a technique which meets the aforesaid requirements. In this technique, two successive scans of a beam of ultraviolet radiation are carried out along a photosensitive portion of the waveguide through a phase mask. During one of the two scans (for example, the first), the phase mask is made to oscillate about its equilibrium position, with an oscillation component parallel to the waveguide, in such a way that the interference fringes have a displacement relative to each other such that the refractive index has an essentially zero envelope as a result of this scanning. This scanning therefore permits control of the local mean refractive index, while the other scanning (carried out with the mask fixed) is used to obtain the desired envelope of the refractive index and, consequently, the desired apodization of the grating.
The two scans are carried out with control of the intensity of the ultraviolet radiation energy which strikes the fibre locally during each of the two scans, achieved by controlling the scanning velocity or by controlling the intensity of the beam of ultraviolet radiation. It is therefore possible to have variable and different velocities, or variable and different beam intensities, during the two scans.
In the case of variable velocities, the variation of the velocity in the oscillating mask scan is selected in such a way as to pre-compensate (or compensate if the scan in question is the second one) the variation of the local mean value of the refractive index associated with the other scan, in such a way that, at the end of the double scan, there is a predetermined variation of the local mean value of the refractive index (for example, a constant value as in the case of the apodized profile in FIG. 2a). During the fixed mask scan, the scanning velocity is selected in such a way that the sinusoidal modulation of the refractive index due to the mask has an envelope equal to the desired apodization. The variation of the local mean value of the refractive index produced by the fixed mask scan depends on the desired apodization.
In the case of variable beam intensities, the criterion for selecting the law of variation of the intensity during the two scans is essentially the same as that for the velocities. The intensity of the laser beam can be controlled, for example, by a variable attenuation of the beam.
Therefore, the oscillatory movement of the phase mask and the control of the intensity of the ultraviolet radiation energy in the fibre during the two scans, achieved by controlling the scanning velocity or, alternatively, by controlling the attenuation of the laser beam, makes it possible to obtain the desired variation of the refractive index.
In a first aspect, the present invention relates to a method for writing a Bragg grating in a waveguide, comprising:
placing a photosensitive waveguide in a writing position in which the said waveguide extends essentially along one axis;
generating a beam of ultraviolet radiation;
executing a first and a second scan of the said beam along the said photosensitive waveguide through a phase mask, to generate interference fringes capable of modifying the refractive index along the said waveguide in a predetermined way; and
moving the said phase mask, during one of the said first and second scans, with an oscillatory motion about one of its equilibrium positions and along a direction of movement lying at an angle of less than 90xc2x0 to the said axis.
Preferably, the method also comprises the variation of the energy of the said ultraviolet radiation along the said portion of photosensitive waveguide during at least one of the said first and second scans.
The said step of varying the energy of the said ultraviolet radiation along the said portion of photosensitive waveguide can comprise varying the scanning velocity of the said beam or, alternatively, varying the intensity of the said beam, preferably by applying a variable attenuation to the said beam.
Preferably, the said direction of movement lies at an angle of more than 0xc2x0 to the said axis, and the method comprises translating the said phase mask along the said direction of movement, before at least one of the said first and second scans, to position the said phase mask at a predetermined distance from the said fibre.
The amplitude and frequency of the said oscillatory movement are advantageously selected in such a way that the said interference fringes have essentially random phases with respect to each other.
The method preferably also comprises transmitting the said beam through a slit of predetermined dimensions.
The said step of executing a first and second scan with the said beam can comprise the steps of deflecting the said beam by means of a mirror and translating the said mirror parallel to the said portion of photosensitive waveguide.
The said second scan is preferably carried out in the direction opposite that of the said first scan.
According to a further aspect, the present invention relates to equipment for writing a Bragg grating in a waveguide, comprising:
an emitter of a beam of ultraviolet radiation;
supporting elements for the said waveguide to place a portion of photosensitive waveguide in a writing position essentially along a predetermined axis and along a path of the said beam;
a phase mask placed on the said path in a position such that it faces the said portion of photosensitive waveguide when the said portion of photosensitive waveguide is in the writing position; and
means for scanning the beam along the said portion of photosensitive waveguide through the said phase mask; and
a movement device carrying the said phase mask and capable of moving the said phase mask with an oscillatory motion in a direction lying at an angle of less than 90xc2x0 to the said axis.
Advantageously, the said movement device is capable of translation in the said direction to position the said phase mask at a predetermined distance from the said fibre.
Additionally, the said movement device preferably comprises a first motorized slide.
The equipment preferably also comprises a screen provided with a slit positioned on the said path before the said phase mask, the said slit having a dimension smaller than the cross section of the said beam.
The said scanning means preferably comprise a second motorized slide carrying the said screen and having a direction of movement orthogonal to the said beam so that the said slit can be positioned at different points of the cross section of the said beam.
Advantageously, the equipment comprises a beam intensity control device, capable of varying the said intensity during the scanning of the beam.
The said beam intensity control device can be an optical attenuator capable of receiving the beam from the said emitter.
The equipment can also advantageously comprise a mirror carried by the said second motorized slide, to deflect the said beam towards the said portion of photosensitive waveguide.